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Moon discovered orbiting tenth planet

This Keck Observatory image shows the planet in the centre, and the moon as a small dot at the 3 o’clock position

(Image: WM Keck Observatory)

The 10th planet and its moon are extremely far from the Sun and the other planets (Artist’s impression: R. Hurt, IPAC)

The tenth planet in the solar system has a moon at least a tenth of its size. The discovery, made on 10 September with an adaptive optics system on the Keck II telescope, will allow astronomers to pin down the mass of both objects.

The tenth planet is not officially a planet – for now its only official designation is 2003 UB313. By convention, its moon – announced by the International Astronomical Union on Sunday – is designated S/2005 (2003 UB313) 1.

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But Mike Brown, the Caltech astronomer who revealed 2003 UB313 in July and nicknamed it Xena after the television warrior princess, is calling the moon Gabrielle, after the princess’s companion.

Moons of “trans-Neptunian” objects – which orbit farther from the Sun than Neptune – are very hard to see because they are faint and appear very close in the sky to the objects they orbit. But Brown began searching for a moon as soon as he found 2003 UB313. “Having a moon is just inherently cool,” he says.

Moons can also reveal vital information about how a planet formed. And astronomers can calculate the masses of both planet and moon by measuring the moon’s orbit and period.

Ghostly companion

So far Brown’s group has only one night of observations of the moon, in which Gabrielle appeared as a faint dot 4.4 magnitudes fainter than Xena, moving with it across the sky. The moon is ten times closer to its planet that our Moon is to Earth, and the pair are 97 times further from the Sun than the Earth is. Viewed from Earth, they are separated by only half an arc-second.

The best estimate of Xena’s size so far is about 2700 kilometres in diameter, about one-fifth the size of the Earth. Gabrielle, which is about a tenth the size of Xena, is about one-eighth the size of our Moon.

An intriguing consequence of the latest discovery is that three of the four largest objects beyond Neptune are now known to have moons, including Pluto and 2003 EL61.

Brown suggests “some very common mechanism must be responsible”. He proposes they were created by collisions between icy trans-Neptunian objects, similar to the collision between the nearly complete Earth and a Mars-sized object that created our Moon.

Further observations over the coming months with Hubble and Keck should reveal more about Xena and Gabrielle. But according to Brown, discovery of the moon will not resolve the issue of how to define a planet, a debate stirred up by the discovery of Xena&colon; “The fact that 2003 UB313 has a moon does not really change the case one way or another.”